U.S. patent application number 09/877231 was filed with the patent office on 2002-03-07 for optical proximity correction method utilizing serifs having variable dimensions.
Invention is credited to Laidig, Thomas, Wampler, Kurt E..
Application Number | 20020028393 09/877231 |
Document ID | / |
Family ID | 24371531 |
Filed Date | 2002-03-07 |
United States Patent
Application |
20020028393 |
Kind Code |
A1 |
Laidig, Thomas ; et
al. |
March 7, 2002 |
Optical proximity correction method utilizing serifs having
variable dimensions
Abstract
A method of forming a mask for optically transferring a
lithographic pattern onto a substrate by use of an optical exposure
tool, where the pattern comprises a plurality of features each of
which has corresponding edges and vertices. The method includes the
steps of forming a serif on a plurality of the vertices contained
in the lithographic pattern, where each of the serifs has a
rectangular shape, and determining the size of each serif
independently on the basis of the length of the feature edges
touching a given vertex, and the horizontal and vertical distance
of the given vertex to the nearest feature edge, wherein the
position of each side of a given serif is independently adjustable
relative to the length of the remaining sides of the given
serif.
Inventors: |
Laidig, Thomas; (Point
Richmond, CA) ; Wampler, Kurt E.; (Sunnyvale,
CA) |
Correspondence
Address: |
MCDERMOTT WILL & EMERY
600 13TH STREET, N.W.
WASHINGTON
DC
20005-3096
US
|
Family ID: |
24371531 |
Appl. No.: |
09/877231 |
Filed: |
June 11, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09877231 |
Jun 11, 2001 |
|
|
|
09592653 |
Jun 13, 2000 |
|
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Current U.S.
Class: |
430/5 ; 430/322;
716/53; 716/55 |
Current CPC
Class: |
G03F 7/70441 20130101;
G03F 1/36 20130101 |
Class at
Publication: |
430/5 ; 716/19;
716/21; 430/322 |
International
Class: |
G06F 017/50; G03F
009/00; G03C 005/00 |
Claims
1. A method of forming a mask for transferring a lithographic
pattern onto a substrate by use of a lithographic exposure
apparatus, said pattern comprising a plurality of features each of
which has corresponding edges and vertices, said method comprising
the steps of: forming a serif on a plurality of said vertices
contained in said lithographic pattern, each of said serifs having
a substantially rectangular shape, and determining the size of each
serif independently on the basis of the length of the feature edges
forming a given vertex, and a first distance and a second distance
from the given vertex to the nearest feature edge, said first
distance and said second distance corresponding to a first
direction and a second direction, respectively, which are
substantially perpendicular to one another and within the plane of
the mask pattern, wherein the position of each side of a given
serif is independently adjustable relative to the positions of the
remaining sides of the given serif.
2. The method of claim 1, further comprising assigning an
adjustment vector to each side of said given serif, said adjustment
vector defining a variation in the location of a corresponding side
of said given serif relative to a default location.
3. The method of claim 1, wherein each side of said serif has a
default position which is selected such that each serif initially
exhibits a substantially square configuration.
4. The method of claim 1, further comprising the step of defining
predetermined rules governing the length of each side of a serif,
said predetermined rules utilizing the results of the determination
of the length of the feature edges forming a given vertex on which
said serif is to be positioned, and the first and second distance
of the given vertex to the nearest feature edge.
5. The method of claim 1, wherein said first and said second
directions correspond to X and Y axes parallel to which the edges
of at least 50% of the mask features extend.
6. The method of claim 1, wherein said vertices comprise convex
vertices and concave vertices, and said serifs comprise positive
serifs and negative serifs, respectively.
7. The method of claim 1, wherein said pattern comprises a
plurality of line end features, each of which has two vertices and
a feature edge, said method comprising the steps of: forming a
serif on each of said vertices contained in a given line end
feature, determining the length of the feature edge, and adjusting
the size of each serif formed on said vertices of said given line
end feature such that the serifs contact each other so as to form
one contiguous edge when said length of said feature edge is less
than a predetermined length.
8. The method of claim 7, wherein the size of each of said serifs
is adjusted such that each of said serifs overlaps 1/2 the length
of said feature edge.
9. The method of claim 7, wherein the maximum amount the size of
each of said serifs can be adjusted by is equal to 1/2 the length
of said feature edge.
10. The method of claim 1, said method comprising the steps of:
forming a serif on at least one of said vertices, said serif having
a rectangular shape, and determining the size of said serif on the
basis of at least the length of the feature edges forming said one
of said vertices, said length of each of said feature edges being
determined by measuring the distance from said one of said vertices
to the closest feature edge which exceeds either a predefined jog
tolerance or a predefined slope tolerance.
11. The method of claim 1, wherein the adjustments to the length of
the sides of a given serif are added together to achieve a total
adjustment.
12. A device manufacturing method comprising the steps of: (a)
providing a substrate that is at least partially covered by a layer
of radiation-sensitive material; (b) providing a projection beam of
radiation using a radiation system; (c) using a mask pattern to
endow the projection beam with a pattern in its cross-section; (d)
projecting the patterned beam of radiation onto a target portion of
the layer of radiation-sensitive material, wherein the mask
employed in step (c) is designed using a method according to claim
1.
13. A device manufactured according to the method of claim 12.
Description
[0001] This application is a continuation in part of application
Ser. No. 09/592,653 filed on Jun. 13, 2000.
FIELD OF THE INVENTION
[0002] The present invention relates to photolithography, and in
particular relates to optical proximity correction methods
utilizing serifs, which are variable in size and position, based on
the location of the particular serif relative to the surrounding
features.
[0003] In addition, the present invention relates to a device
manufacturing method using a lithographic apparatus comprising:
[0004] a radiation system for providing a projection beam of
radiation;
[0005] a mask table for holding a mask, serving to pattern the
projection beam;
[0006] a substrate table for holding a substrate;
[0007] a projection system for projecting the patterned projection
beam onto a target portion of the substrate.
BACKGROUND OF THE INVENTION
[0008] Lithographic projection apparatus (tools) can be used, for
example, in the manufacture of integrated circuits (ICs). In such a
case, the mask contains a circuit pattern corresponding to an
individual layer of the IC, and this pattern can be imaged onto a
target portion (e.g. comprising one or more dies) on a substrate
(silicon wafer) that has been coated with a layer of
radiation-sensitive material (resist). In general, a single wafer
will contain a whole network of adjacent target portions that are
successively irradiated via the projection system, one at a time.
In one type of lithographic projection apparatus, each target
portion is irradiated by exposing the entire mask pattern onto the
target portion in one go; such an apparatus is commonly referred to
as a wafer stepper. In an alternative apparatus--commonly referred
to as a step-and-scan apparatus--each target portion is irradiated
by progressively scanning the mask pattern under the projection
beam in a given reference direction (the "scanning" direction)
while synchronously scanning the substrate table parallel or
anti-parallel to this direction; since, in general, the projection
system will have a magnification factor M (generally <1), the
speed V at which the substrate table is scanned will be a factor M
times that at which the mask table is scanned. More information
with regard to lithographic apparatus as here described can be
gleaned, for example, from U.S. Pat. No. 6,046,792, incorporated
herein by reference.
[0009] In a manufacturing process using a lithographic projection
apparatus, a mask pattern is imaged onto a substrate that is at
least partially covered by a layer of radiation-sensitive material
(resist). Prior to this imaging step, the substrate may undergo
various procedures, such as priming, resist coating and a soft
bake. After exposure, the substrate may be subjected to other
procedures, such as a post-exposure bake (PEB), development, a hard
bake and measurement/inspection of the imaged features. This array
of procedures is used as a basis to pattern an individual layer of
a device, e.g. an IC. Such a patterned layer may then undergo
various processes such as etching, ion-implantation (doping),
metallization, oxidation, chemo-mechanical polishing, etc., all
intended to finish off an individual layer. If several layers are
required, then the whole procedure, or a variant thereof, will have
to be repeated for each new layer. Eventually, an array of devices
will be present on the substrate (wafer). These devices are then
separated from one another by a technique such as dicing or sawing,
whence the individual devices can be mounted on a carrier,
connected to pins, etc. Further information regarding such
processes can be obtained, for example, from the book "Microchip
Fabrication: A Practical Guide to Semiconductor Processing", Third
Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN
0-07-067250-4, incorporated herein by reference.
[0010] The lithographic tool may be of a type having two or more
substrate tables (and/or two or more mask tables). In such
"multiple stage" devices the additional tables may be used in
parallel, or preparatory steps may be carried out on one or more
tables while one or more other tables are being used for exposures.
Twin stage lithographic tools are described, for example, in U.S.
Pat. No. 5,969,441 and WO 98/40791, incorporated herein by
reference The photolithographic masks referred to above comprise
geometric patterns corresponding to the circuit components to be
integrated onto a silicon wafer. The patterns used to create such
masks are generated utilizing CAD (computer-aided design) programs,
this process often being referred to as EDA (electronic design
automation). Most CAD programs follow a set of predetermined design
rules in order to create functional masks. These rules are set by
processing and design limitations. For example, design rules define
the space tolerance between circuit devices (such as gates,
capacitors, etc.) or interconnect lines, so as to ensure that the
circuit devices or lines do not interact with one another in an
undesirable way.
[0011] These design rule limitations are typically referred to as
"critical dimensions" (CD). A critical dimension of a circuit can
be defined as the smallest width of a line or the smallest space
between two lines. Thus, the CD determines the overall size and
density of the designed circuit.
[0012] Of course, one of the goals in integrated circuit
fabrication is to faithfully reproduce the original circuit design
on the wafer (via the mask). Another goal is to use as much of the
semiconductor wafer real estate as possible. As the size of an
integrated circuit is reduced and its density increases, however,
the CD of its corresponding mask pattern approaches the resolution
limit of the optical exposure tool. The resolution for an exposure
tool is defined as the minimum feature that the exposure tool can
repeatedly expose on the wafer. The resolution value of present
exposure equipment often constrains the CD for many advanced IC
circuit designs.
[0013] As the critical dimensions of the circuit layout become
smaller and approach the resolution value of the exposure tool, the
correspondence between the mask pattern and the actual circuit
pattern developed on the photoresist layer can be significantly
reduced. The degree and amount of differences in the mask and
actual circuit patterns depends on the proximity of the circuit
features to one another.
[0014] Accordingly, pattern transference problems are referred to
as "proximity effects." Proximity effects occur when very closely
spaced circuit pattern features are lithographically transferred to
a resist layer on a wafer. The light waves of the closely spaced
circuit features interact, thereby distorting the final transferred
pattern features.
[0015] Another common proximity effect problem caused by
approaching the resolution limit of the exposure tool is that the
corners of the photoresist (both concave and convex) tend to
overexpose or underexpose due to a concentration or lack of energy
at each of the corners. For example, during exposure to light or
radiation the photoresist layer integrates energy contributions
from all surrounding areas. Thus, the exposure dose in one vicinity
of the wafer is affected by the exposure dose in neighboring
vicinities.
[0016] Because a corner region in a mask pattern lacks neighboring
regions, the exposure dose to a corner of the photoresist layer
will always be less than the exposure dose to the main body of the
layer. The corners of the developed photoresist pattern, therefore,
tend to be rounded, rather than angular, due to the fact that less
energy has been delivered to the corners than to the other areas of
the masked pattern. In small, dense integrated circuits, such as
VLSI circuits, these rounding effects can cause a significant
degradation to the circuit's performance. Moreover, rounding
results in a loss of contact surface area, which correspondingly
reduces the total area available for conduction and accordingly
results in an undesirable increase in contact resistance.
[0017] To help overcome the significant problem of proximity
effects, a number of techniques are used to add sub-lithographic
features to mask patterns. Sub-lithographic features have
dimensions less than the resolution of the exposure tool, and
therefore do not transfer to the photoresist layer. Instead,
sub-lithographic features interact with the original mask pattern
and compensate for proximity effects, thereby improving the final
transferred circuit pattern.
[0018] Examples of such sub-lithographic features are scattering
bars and anti-scattering bars, such as disclosed in U.S. Pat. No.
5,821,014 (incorporated herein by reference), which are added to
mask patterns to reduce differences between features within a mask
pattern caused by proximity effects. Another technique used to
improve circuit pattern transference from design to wafer is to add
features to the mask pattern called "serifs", such as disclosed in
U.S. Pat. No. 5,707,765 (incorporated herein by reference). Serifs
are typically sub-lithographic square-shaped features positioned on
each corner of a circuit feature. The serifs serve to "sharpen" the
corners in the final transferred design on the wafer, thereby
improving the correspondence between the actual circuit design and
the final transferred circuit pattern on the photoresist layer.
Serifs are also used at the intersection areas of differing circuit
patterns in order to compensate for the distortion factor caused by
the intersection of two different circuit patterns.
[0019] In particular, it is known to include "positive" serifs at
convex vertices so as to adjust the exposure energy of the mask at
the vertex to prevent loss of the corner, and to include "negative"
serifs at concave vertices. The "negative" serifs essentially
remove a portion of the mask pattern at the concave vertex so as to
attempt to maintain an accurate representation of the concave
vertex in the final pattern formed on the wafer.
[0020] However, prior to the present invention, known methods of
applying serifs to feature patterns utilized serifs having a
predetermined, non-adjustable size. For example, each positive
serif contained in the mask pattern had the same dimension, and
similarly, each negative serif contained in the mask pattern had
the same dimension. Furthermore, in such known systems, the
determination of whether or not a serif would be provided in a
specific location was determined based solely on the distance of
the serif to the next closest serif. In other words, if there was
sufficient room to accommodate the predetermined, singular size
serif without causing interference (i.e. bridging) with the closest
adjacent serif, the serif would be included in the mask design.
However, if inclusion of the serif would result in interference
with another serif, both serifs were simply cancelled from the
pattern, resulting in an undesirable degradation in the final mask
pattern and subsequent feature printing.
[0021] Such known methods typically defined a minimum allowable
distance between a serif and the adjacent serif. As stated, if two
serifs were separated by less than the minimum allowable distance,
the serifs simply cancelled one another (i.e. both serifs were
omitted).
[0022] While such known methods of placing serifs are acceptable
when the minimum distance between features is sufficiently large,
as today's photolithography techniques are continually reducing the
minimum distance required between features, the foregoing known
method can result in an unacceptable elimination of a significant
number of serifs, which correspondingly results in an undesirable
reduction in the accuracy of the reproduction of the desired
circuit on the wafer.
[0023] For example, in the case of a line end having two convex
vertices, it is necessary to place positive serifs on each vertex
so as to prevent undesirable shortening of the line formed on the
wafer. However, due to ever decreasing line widths, in the
foregoing serif placement method, the serifs on the two vertices
would simply cancel each other because the serifs would be too
close to one another. As stated, this results in an undesirable
line shortening in the final pattern.
[0024] In addition, known serif placement methods do not provide a
mechanism for determining whether or not a serif would result in
interference with an adjacent feature. This is due to the fact that
the CD for typical circuits would readily allow for placement of
the serif without any resulting interference. However, as the CD of
circuit designs is continually decreasing, it is increasingly
likely that positive serifs may interfere with adjacent
features.
[0025] Accordingly, there exists a need for a method of performing
optical proximity correction utilizing serifs that allows for the
flexible design of serifs, such that serifs located proximate one
another that fail to satisfy a minimum distance requirement are not
simply cancelled from the mask pattern. In addition, there is a
need for an optical proximity correction method that verifies and
prevents interference between serifs and adjacent features.
SUMMARY OF THE INVENTION
[0026] In an effort to solve the aforementioned needs, it is an
object of the present invention to provide an optical proximity
correction method utilizing serifs that allows for the modification
of the dimension of each serif, as well as the position of each
serif, on an individual basis so as to allow adjacent serifs that
initially fail to satisfy a minimum distance requirement to be
modified in size and/or position so as to meet the minimum distance
requirement.
[0027] It is also an object of the present invention to provide an
optical proximity correction method that verifies and prevents
interference between serifs and adjacent features.
[0028] More specifically, in a first embodiment, the present
invention relates to a method of forming a mask for transferring a
lithographic pattern onto a substrate by use of a lithographic
exposure apparatus (tool), where the pattern comprises a plurality
of features each of which has corresponding edges and vertices. The
method comprises the steps of forming serifs on a plurality of the
vertices contained in the lithographic pattern, where each of the
serifs has a (substantially) rectangular shape, and determining the
size of each serif independently on the basis of the lengths of the
feature edges forming a given vertex, and the "horizontal" and
"vertical" distance of the given vertex to the nearest feature
edge, wherein the position of each side of a given serif is
independently adjustable relative to the position of the remaining
sides of the given serif. As here employed, the terms "horizontal"
and "vertical" refer to mutually orthogonal directions that lie
within the plane of the mask pattern; conventionally, they
correspond to the X and Y axes parallel to which the edges of most
(if not all) of the mask features extend
[0029] In a second embodiment, the present invention relates to a
method of forming a mask for transferring a lithographic pattern
onto a substrate by use of a lithographic exposure apparatus, where
the pattern comprises a plurality of features each of which has
corresponding edges and vertices. The method comprises the steps of
forming a serif on at least one of the vertices contained in the
lithographic pattern, and determining the size of the serif on the
basis of at least the length of the feature edges forming said one
of the vertices, where the length of each of the feature edges is
determined by measuring the distance from said one of the vertices
to the closest feature edge that exceeds either a predefined jog
tolerance or a predefined slope tolerance.
[0030] In a third embodiment, the present invention relates to a
method of forming a mask for transferring a lithographic pattern
onto a substrate by use of a lithographic exposure apparatus, where
the pattern comprises a plurality of line end features, each of
which has two vertices and a feature edge. The method comprises the
steps of forming a serif on each of the vertices contained in a
given line end feature, determining the length of the feature edge,
and adjusting the size of each serif formed on the vertices of the
given line end feature such that the serifs contact each other so
as to form one contiguous surface when the length of the feature
edge is less than a predetermined size.
[0031] As described in further detail below, the present invention
provides significant advantages over the prior art. Most
importantly, the optical proximity method of the present invention
provides for individual and flexible control of each serif so as to
allow the size and position of each serif to be adjustable. As
such, it is possible to modify the dimensions of serifs during the
mask design process so that serifs that would otherwise be
eliminated from the design satisfy minimum distance requirements.
As a result, the present invention minimizes the elimination of
serifs from the mask design, and the corresponding degradation in
the pattern formed on the wafer.
[0032] Although specific reference may be made in this text to the
use of the invention in the manufacture of ICs, it should be
explicitly understood that the invention has many other possible
applications. For example, it may be employed in the manufacture of
integrated optical systems, guidance and detection patterns for
magnetic domain memories, liquid-crystal display panels, thin-film
magnetic heads, etc. The skilled artisan will appreciate that, in
the context of such alternative applications, any use of the terms
"reticle", "wafer" or "die" in this text should be considered as
being replaced by the more general terms "mask", "substrate" and
"target portion", respectively.
[0033] In the present document, the terms "radiation" and "beam"
are used to encompass all types of electromagnetic radiation,
including ultraviolet radiation (e.g. with a wavelength of 365,
248, 193, 157 or 126 nm) and EUV (extreme ultra-violet radiation,
e.g. having a wavelength in the range 5-20 nm).
[0034] The term mask as employed in this text may be broadly
interpreted as referring to generic patterning means that can be
used to endow an incoming radiation beam with a patterned
cross-section, corresponding to a pattern that is to be created in
a target portion of the substrate; the term "light valve" can also
be used in this context. Besides the classic mask (transmissive or
reflective; binary, phase-shifting, hybrid, etc.), examples of
other such patterning means include:
[0035] A programmable mirror array. An example of such a device is
a matrix-addressable surface having a viscoelastic control layer
and a reflective surface. The basic principle behind such an
apparatus is that (for example) addressed areas of the reflective
surface reflect incident light as diffracted light, whereas
unaddressed areas reflect incident light as undiffracted light.
Using an appropriate filter, the said undiffracted light can be
filtered out of the reflected beam, leaving only the diffracted
light behind; in this manner, the beam becomes patterned according
to the addressing pattern of the matrix-adressable surface. The
required matrix addressing can be performed using suitable
electronic means. More information on such mirror arrays can be
gleaned, for example, from U.S. Pat. Nos. 5,296,891 and 5,523,193,
which are incorporated herein by reference.
[0036] A programmable LCD array. An example of such a construction
is given in U.S. Pat. No. 5,229,872, which is incorporated herein
by reference.
[0037] Additional advantages of the present invention will become
apparent to those skilled in the art from the following detailed
description of exemplary embodiments of the present invention.
[0038] The invention itself, together with further objects and
advantages, can be better understood by reference to the following
detailed description and the accompanying schematic drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0039] FIG. 1 illustrates an exemplary feature having both a
positive serif and a negative serif disposed thereon.
[0040] FIG. 2a illustrates a U-edge as defined by the present
invention.
[0041] FIG. 2b illustrates an S-edge as defined by the present
invention.
[0042] FIG. 3 illustrates how the variables utilized in the
determination of the size and position of a serif are computed in
accordance with the present invention.
[0043] FIG. 4 illustrates the adjustment vectors for manipulating
the size and position of the serifs according to the present
invention.
[0044] FIG. 5 illustrates an example of how serifs are adjusted in
size in accordance with an exemplary adjustment rule in accordance
with the present invention.
[0045] FIG. 6 illustrates an example of how serifs are repositioned
in accordance with an exemplary adjustment rule in accordance with
the present invention.
[0046] FIG. 7 illustrates an example of how the present invention
can be utilized to prevent serif loss on vertices near small jogs
in feature edges.
[0047] FIG. 8 illustrates a lithographic projection apparatus.
[0048] In the Figures, like reference symbols indicate like
parts.
DETAILED DESCRIPTION OF THE INVENTION
[0049] In accordance with the optical proximity correction method
of the present invention, each edge of a given serif (both positive
and negative) is adjustable relative to every other edge of the
given serif so as to allow the overall dimension and the position
of the serif to be modified. In addition, each serif can be
modified independently of every other serif. The modification of
serifs is governed by predefined rules, which are selected/defined
by the user in accordance with the feature pattern to be generated.
Of course, such rules would vary in accordance with different
circuit design features; for example, minimum allowable CD. It is
noted that the method of the present invention is typically
embodied/incorporated into a program executable on a CAD/EDA system
utilized to generate the desired mask pattern.
[0050] FIG. 1 illustrates both a positive serif 12 formed on a
convex vertex and a negative serif 13 formed on a concave vertex.
In accordance with the present invention, initially each positive
and negative serif 12, 13 is defined as a nominal square serif. As
shown, the positive serif 12 is defined by an overall width psw, an
overlap pso and an extension pse. As is clear from the figure, pso
corresponds to the portion of the positive serif 12 extending over
the mask feature 11, pse corresponds to the portion of the positive
serif 12 extending beyond the mask feature 11, and psw=pso+pse.
With regard to the negative serif 13, nso corresponds to the
portion of the negative serif 13 not extending over the mask
feature 11, nse corresponds to the portion of the negative serif 13
extending over the mask feature 11, and nsw=nso+nse. In the current
embodiment, the default values of both positive and negative serifs
are rotationally symmetric. In other words, if one of the serifs
were rotated by 90 degrees, the overlap and extension values of the
serif would remain the same.
[0051] It is noted that it is permissible to define the nominal (or
default) dimensions of both positive and negative serifs
differently from one another. In addition, while the initial shapes
of both positive and negative serifs are shown as squares, it is
permissible to utilize a different shape or size for the default
value of the serifs.
[0052] The method of the present invention also classifies the
edges forming a vertex on which a serif is to be placed into one of
two categories. Specifically, each edge forming a vertex is
classified as either a "U-edge" or an "S-edge". FIG. 2a illustrates
an example of a U-edge. As shown, the U-edge 15 has a convex vertex
14 formed on each end thereof. A common U-edge is a line end.
Similarly, an edge that has concave vertices on both ends is also a
U-edge. FIG. 2b illustrates an S-edge 16. As shown, the S-edge 16
has a convex vertex 17 on one end and a concave vertex 18 on the
other end. It is noted that an S-edge represents what is referred
to as a "jog" (i.e. step). Specifically, referring to FIG. 2b, the
length of the S-edge represents the depth of the jog. In some
instances, as explained in detail below, if the depth of the jog is
below a predefined minimum, the S-edge is ignored and the edges
connected to the S-edge are treated as a single continuous
edge.
[0053] As indicated above, in the method of the present invention,
the size and position of each serif are affected by the
surrounding/adjacent features contained in the mask pattern.
Accordingly, it is necessary to determine the surrounding features
when placing and sizing a serif for a given vertex. In the current
embodiment, each serif generated for a given vertex can be
influenced by up to four independent variables. Specifically, the
four variables are: 1) the length of the first edge touching the
vertex (i.e. either a U or S-edge), 2) the length of the second
edge touching the vertex, 3) the "horizontal" distance from the
vertex to the nearest feature edge and 4) the "vertical" distance
from the vertex to the nearest feature edge (using the earlier
definition of the terms "horizontal" and "vertical"). The results
of the four foregoing variables are utilized to determine the size
and position of a given serif in accordance with predefined rules,
which may vary from design to design.
[0054] FIG. 3 illustrates how the four variables would be measured
for both a convex vertex "A" and a concave vertex "B". Referring to
FIG. 3, the portion of the mask pattern disclosed therein contains
three features 21, 22 and 23. Feature 21 comprises a first S-edge
24, a second S-edge 25 (both indicated by "sed") and a U-edge 26
(indicated by "ued"). S-edges 24 and 25 are joined at vertex B,
while S-edge 25 and U-edge 26 are joined at vertex A. With regard
to vertex A, the lengths of the edges touching vertex A comprise
the length of edge 25 and the length of edge 26. The "horizontal"
distance from vertex A to the nearest feature edge, is the distance
to feature 23. The "vertical" distance from vertex A to the nearest
feature edge, is the distance to feature 22. With regard to vertex
B, the lengths of the edges touching vertex B comprise the length
of edge 24 and the length of edge 25. The "horizontal" distance
from vertex B to the nearest feature edge, is the distance
(represented by line 27) to the outside edge of feature 21. The
"vertical" distance from vertex B to the nearest feature edge, is
the distance (represented by line 28) to the outside edge of
feature 21. As is clear from the foregoing, it is noted that for
convex vertices, the intervals (indicated by "int") are measured
from the vertex to the nearest adjacent out-facing edge, and for
concave vertices, the intervals are measured from the vertex to the
nearest "in-facing" edge of the same feature.
[0055] Furthermore, when determining interval measurement for a
convex vertex, a .+-.45 degree field of view is considered when
searching for an adjacent feature edge. It is noted that in the
current embodiment, even in the case where the adjacent feature
edge is on a diagonal line to the vertex under consideration, the
distance or interval length utilized in the calculation process is
defined as the perpendicular distance to the facing edge. Vertex C
of FIG. 3 illustrates the foregoing point. Specifically, the
vertical distance from vertex C to feature 22 is the perpendicular
distance from U-edge 31 to the line defined by the bottom edge of
feature 22, as represented by line 32.
[0056] In accordance with the present invention, each of the four
edges of any given serif may be adjusted in response to any of the
four independent measurements (i.e. variables) noted above and
illustrated in FIG. 3. The amount that a particular serif edge
moves away from the default value of the serif is the sum of the
components specified in the predetermined rules relating to the
particular design and process being utilized. Examples of how serif
edges may be adjusted are set forth below. It is noted however,
that the present invention is not limited to the specific examples
shown. Indeed, when or how to adjust a given serif will depend on
numerous factors, many of which are design and process dependent,
and as such, would vary from application to application.
[0057] FIG. 4 illustrates how a positive serif 41 can be adjusted
in accordance with the present invention. While not shown, negative
serifs are adjustable in a similar manner. Referring to FIG. 4, as
stated above each of the four edges of a serif may be adjusted
independently of the other edges. In the current embodiment, the
present invention utilizes four adjustment vectors, each of which
corresponds to a given edge of the serif, to designate how a
particular edge should be moved relative to the default dimensions
of the serif. In FIG. 4, the four adjustment vectors are diagrammed
relative to the length of the S-edge 42. As shown, the four
adjustment vectors are designated: 1) paex, 2) peex, 3) paol and 4)
peol. The vector paex corresponds to the distance the upper edge 43
of the serif 41 extends from its default value position, whereas
paol corresponds to the distance the bottom edge 44 of the serif 41
extends from its default position. Similarly, peol corresponds to
the distance the left-side edge 45 of the serif 41 extends from its
default position, and peex corresponds to the distance the
right-side edge 46 of the serif 41 extends from its default
position. Although not shown in FIG. 4, it is noted that negative
values for any of the four adjustment vectors indicate a decrease
in serif size from the default value. It is further noted that
while positive values have been utilized to indicate an increase in
serif size, and negative values a decrease, clearly the opposite is
also possible.
[0058] Accordingly, assuming a positive value for paex, and default
values for the remaining three adjustment vectors, the result would
be a serif having an increased size and a rectangular, but
non-square shape.
[0059] As a result of defining individual vectors for each edge of
each given serif to be included in the mask pattern, the present
invention allows for individual control of the size and placement
of each serif. For example, referring to FIG. 4., assuming an
adjacent feature (not shown) was positioned too closely to the top
edge 43 of serif 41 such that the minimum spacing rule was
violated, it is possible to retract the top edge 43 of serif 41, by
applying the appropriate negative value of paex, thereby reducing
the size of the serif, such that the minimum spacing rule is no
longer violated. Alternatively, it is also possible to retract the
entire serif 41 from the feature until the minimum spacing rule is
satisfied. This solution allows a serif of the same size to be
utilized, with the serif shifted in position. Of course, other
variations are also possible. The important aspect is that the
serif is not eliminated from the mask pattern, and as a result,
degradation of the pattern subsequently printed utilizing the mask
is prevented.
[0060] It is noted that the user defines the rules regarding how
and when serif dimensions should be modified in response to data
obtained regarding the four independent variables noted above. The
rules are then typically stored in the CAD/EDA system utilized to
generate the mask pattern. When a given rule is applicable, the
CAD/EDA system automatically adjusts the size of the given serif in
accordance with the given rule. Examples of such rules are set
forth below.
[0061] Table I set forth below lists exemplary base parameters
related to the computation and adjustment of serif size and
dimension. Referring to Table I, most of the base parameters have
been discussed above. It is noted that the term "nominal" is
utilized to designate the default dimensions of the serif. As
stated above, the default dimensions are predetermined, and can be
varied from application to application.
1TABLE I Parameter Comments serif_style = flexible Adjustable
serifs/hammerheads serif_width Nominal size of positive and
negative serifs psw (ps_width) Nominal size of positive serifs nsw
(ns_width) Nominal size of negative serifs serif_ext Nominal serif
extension beyond primary figure corners pse (ps_ext) Nominal
positive serif extension nse (ns_ext) Nominal negative serif
extension serif_overlap Nominal serif overlap of primary figure
corner pso (ps_overlap) Nominal positive serif overlap nso
(ns_overlap) Nominal negative serif overlap sjt
(serif_jog_tolerance) Maximum jog in U-edge or S-edge sst
(serif_slope_tolerance) Maximum slope of non-rectilinear edge
serif_min_width Minimum serif width ps_min_width Minimum positive
serif width ns_min_width Minimum negative serif width serif_adj =
{sublist} Positive and negative serif adjustment rule ps_adj =
{sublist) Positive serif adjustment rule ns_adj = {sublist)
Negative serif adjustment rule do_serifs Boolean operation: enables
serif synthesis
[0062] With regard to the parameters not previously discussed,
serif_width, serif_ext and serif_overlap are utilized when the
default values for both the positive and negative serifs are equal.
It is noted that in the current embodiment, serif_overlap is
automatically limited to half the edge length. As explained below,
sjt (serif_jog_tolerance) and sst (serif_slope_tolerance) are
utilized to define whether or not an S-edge in jog will be treated
as a separate edge or ignored. The parameter serif_min_width
defines the minimum width of both positive and negative serifs,
whereas ps_min_width and ns_min_width define the minimum width of
positive and negative serifs, respectively. It is noted that
serif_min_width is only utilized if both ps_min_width and
ns_min_width are identical. The parameters serif_adj, ps_adj and
ns_adj each correspond to a rule statement that acts on a given
serif in response to one of the four independent context variables
noted above. The command do_serifs corresponds to a switch which
enables or disables serif creation; ndo_serifs is utilized to
disable.
[0063] Table II sets forth sub-option parameters pertaining to the
measurements associated with individual serifs and the nearest
adjacent feature as discussed above, as well as the extensions
utilized to adjust the size of the individual serifs. It is noted
that each of the sub-option parameters set forth in Table II can be
either a single value or a value range. When a range is indicated,
the value varies linearly between the two stated extremes. It is
noted that when a range of values is specified for a context
variable (e.g. sed=(0.1:0.3)), the adjustment response may also be
a range (e.g. paol=(-0.2:0)), and linear interpolation can be
utilized for intermediate dimensions.
2TABLE II Parameter Variable Type Comments ued Independent U-edge
length sed Independent S-edge length mt (interval) Independent
Distance from corner to adjacent figure edge paex Dependent Serif
extension parallel to edge or interval paol Dependent Serif overlap
parallel to edge or interval peex Dependent Serif extension
perpendicular to edge or interval peol Dependent Serif overlap
perpendicular to edge or interval
[0064] Examples of how the present invention can be utilized to
modify serifs are now provided. First, serif cancellation occurs
when the adjusted serif size falls below the value of
serif_min_width. This can occur, for example, when a side of a
serif is retracted in an effort to satisfy the minimum separation
requirement between the serif and an adjacent feature, and the
width of the serif which satisfies the minimum separation
requirement results in a serif having a width less than
serif_min_width. In such an instance, the serif is cancelled. A
cancellation can also be prescribed in a rule, for example, by
specifying an amount of minus infinity for any one of the four
serif edge adjustments, which simply reduces the length of the
associated edge to zero, thereby eliminating the serif from the
mask pattern. It is noted that while one of the goals of the
present invention is to prevent or minimize the cancellation of
serifs, cancellation of serifs is still possible when necessary to
satisfy a given rule defined by the user.
[0065] FIG. 5 illustrates another example of how the present
invention can be utilized to adjust serifs disposed on U-edges. As
mentioned above, depending on the width of a given line, it is
often desirable to form one continuous serif (generally referred to
as a "hammer head") on the corresponding line so as to prevent
unwanted shortening of the line during fabrication. In the example
illustrated in FIG. 5, the applied rules regarding serif adjustment
are as follows:
[0066] psw=0.10,
[0067] pso=0.03,
[0068] ps_adj={sed=(0:0.16), paex=-infinity},
[0069] ps_adj={ued=(0:0.16), paol=infinity}, and
[0070] do_serifs
[0071] Referring to FIG. 5, there is one feature 51 having a main
horizontal portion 52 and four vertical portions 53-56. In
accordance with the foregoing rules, first only positive serifs
will be applied to convex vertices. The nominal width of each serif
will be 0.10 and the nominal overlap of the serif on the mask
feature will be 0.03. However, the adjustment rules modify the
serifs under the following conditions. Specifically, if the length
of an S-edge touching a convex vertex is within the range 0-0.16,
the serif is omitted (by making paex equal minus infinity). In
addition, if the length of a U-edge touching a convex vertex is
within the range 0-0.16, the serif is extended over half of the
length of the edge (by making paol equal infinity). It is noted
that even though the overlap is designated to be infinity, the
current embodiment limits the maximum overlap to 1/2 the length of
the corresponding edge.
[0072] Applying the foregoing exemplary rules to the features
illustrated in FIG. 5, a first serif 61 and a second serif 62 are
placed on the vertices of U-edge 63. In accordance with the rules,
each serif 61 and 62 has an overall width of 0.10 and overlap of
0.03. It is noted that U-edge 63 has a length greater than
0.16.
[0073] Next, the convex vertex formed by S-edge 64 does not have a
serif disposed thereon, because the length of S-edge 64 is only
0.16. As such, paex is set to minus infinity, thereby resulting in
the omission of a serif from the convex vertex formed by S-edge 64.
It is noted that the foregoing rules do not indicate or command
placement of negative serifs; as such, no negative serifs appear in
FIG. 5.
[0074] Continuing, referring to vertical portion 54 containing
U-edge 65 having two convex vertices, similar to serifs 61 and 62,
a positive serif 66, 67, each having an overall width of 0.10 and
overlap of 0.03, is placed on each vertex. It is noted that the
length of U-edge 65 is greater than 0.16.
[0075] However, vertical portion 55 has a width of 0.16, and
therefore U-edge 68 has a length of 0.16. As such, the ps_adj rule
applicable to U-edges is applied. As a result, each serif 69, 70
formed on the convex vertices at the ends of U-edge 68 is formed so
as to extend over half of U-edge 68. This results in a continuous
serif or hammerhead (formed by two separate serifs) extending over
U-edge 68. It is noted that the formation of a hammerhead serif
instead of two distinct corner serifs in such a situation is
exceedingly beneficial, because it removes the difficulty of
fabricating the small gap between serifs on the mask, while having
nearly identical effect during printing.
[0076] Referring to vertical portion 56, it is noted that U-edge 71
has a length of 0.16 and S-edge 72 has a length of 0.16. As such,
both rules set forth above regarding ps_adj of S-edges and U-edges
are applied. Specifically, as S-edge 72 has a length of 0.16, paex
is set to equal minus infinity, which results in the omission of a
serif on the convex vertex formed by S-edge 72. In addition, as
U-edge 71 has a length of 0.16, paol is set to equal infinity,
which results in serif 73 extending half the distance of U-edge
71.
[0077] Finally, turning to U-edge 75, it is noted that serif 73 is
not also extended in the "vertical" direction along U-edge 75
because the length of U-edge 75 exceeds 0.16, and therefor the
ps_adj rule is not applicable. Serif 76 is formed on the lower
convex vertex of U-edge 75 in the manner similar to serif 61.
[0078] FIG. 6 illustrates an example of how the present invention
can be utilized to reposition serifs based on the proximity of
adjacent features. In the example illustrated in FIG. 6, the
applied adjustment rules are as follows:
[0079] serif_width=0.10,
[0080] serif_ext=0.07
[0081] serif_adj (a)={int=(0.32:0.16), paex=(0:-0.07),
paol=(0:0.07)},
[0082] serif_adj (b)={int=(0.16:0), paex=-0.07, paol=0.07}, and
[0083] do_serifs
[0084] In accordance with the foregoing rules, in the event the
"interval" between the vertex on which serif is to be placed and
the facing edge of an adjacent feature is between 0.16 and 0.32,
the corresponding serif will be shifted in position by decreasing
the par_ext by a linearly-interpolated value in the range of
0:-0.07 and increasing the par_olap by a value in the range 0:0.07.
Alternatively, in the event the "interval" between the vertex on
which the serif is to be placed and the adjacent feature edge is
between 0.0 and 0.16, the corresponding serif will be shifted in
position by decreasing the par_ext by a value of -0.07 and
increasing the par_olap by a value of 0.07 remaining
"fully-retracted".
[0085] Referring to FIG. 6, applying the foregoing rules to the
exemplary features set forth in FIG. 6, feature 81 is separated
from feature 80 by 0.32. As such, serif_adj (a) is applicable, and
both serifs 85 are retracted by 0.0 in accordance with the rule. It
is noted that serif_adj (a) provides a range (i.e. interval) over
which the rule is applicable, as well as a range for adjusting the
serifs. As the distance of 0.32 is the starting point of
application of the rule, and has a corresponding adjustment of 0,
serifs 85 are not adjusted. As noted above, when a range/interval
is applied, in the present embodiment, linear interpolation is
utilized to determine how much the serif should be adjusted.
Continuing, feature 82 is separated from feature 80 by 0.24. Once
again, serif_adj (a) is applicable, and therefore serifs 86 are
retracted approximately 0.035 in accordance with the rule. Feature
83 is separated from feature 80 by 0.16. As such, serif_adj (b) is
applicable, and therefore serifs 86 are retracted approximately
0.07, which represents the maximum retraction in the current
example. As shown, the upper edges of serifs 87 are substantially
flush with the upper edge of feature 87. Finally, feature 84 is
separated from feature 80 by 0.10. Once again, serif_adj (b) is
applicable, and therefore serifs 88 are retracted approximately
0.07. It is again noted that the foregoing is merely an
illustrative example of one possible rule.
[0086] FIG. 7 illustrates how the present invention can be utilized
to prevent serif loss on vertices near small jogs in feature edges.
More specifically, sjt and sst parameters are provided to control
how small jogs influence the computed U-edge and S-edge length.
Such jogs may arise in the original circuit data, as a result of
fine biasing for proximity correction, or as a result of
decomposing diagonal feature edges into stair steps.
[0087] Referring to FIG. 7, in accordance with the current
embodiment, the span from vertex A to vertex B will be treated as a
single U-edge provided that all jogs in the edge fall within the
predefined jog and slope tolerances. If either tolerance is reduced
to overlap one of the jogs, the measured edge length will span from
vertex A to the first jog that falls out of bounds with the
tolerance. As shown in FIG. 7, referring to vertex A, the length of
a given jog is measured by the perpendicular distance from the
U-edge 91 touching vertex A to the surface 92, which extends
parallel to surface 91, and which is formed as a result of the jog
93. If the length of the jog 93 exceeds the predefined
jog_tolerance, then the jog 93 is treated as an actual edge, and is
not ignored. Referring again to vertex A, to determine if the sst
is violated, it is necessary to measure angle .theta. and verify
that it is less than a predetermined value. As shown in FIG. 7,
.theta. associated with a given vertex (e.g. A) is measured by
determining the angle formed by the line intersecting the given
vertex, and the upper corner of the nearest jog to A.
[0088] It is noted that in accordance with the present embodiment,
the edge length corresponding to a given vertex is measured
independently for each vertex touching the given edge. Accordingly,
based on a given set of jog and slope tolerances, it is possible
that the length of A's U-edge 91 might extend to vertex B, but B's
U-edge 91 would not extend to vertex A. Parameter sjt is typically
set high enough to ignore the maximum jog that could be produced by
fine-bias OPC correction (e.g. typical values are 5%-20% of the
minimum width CD). In addition, in the current embodiment, edge jog
consolidation will stop at the first deviation in the edge's
direction that exceeds .+-.90 degrees, even if it remains
positionally within the jog and slope tolerances.
[0089] FIG. 8 schematically depicts a lithographic projection
apparatus suitable for use with a mask designed with the aid of the
current invention. The apparatus comprises:
[0090] a radiation system Ex, IL, for supplying a projection beam
PB of radiation. In this particular case, the radiation system also
comprises a radiation source LA;
[0091] a first object table (mask table) MT provided with a mask
holder for holding a mask MA (e.g. a reticle), and connected to
first positioning means for accurately positioning the mask with
respect to item PL;
[0092] a second object table (substrate table) WT provided with a
substrate holder for holding a substrate W (e.g. a resist-coated
silicon wafer), and connected to second positioning means for
accurately positioning the substrate with respect to item PL;
[0093] a projection system ("lens") PL (e.g. a refractive,
catoptric or catadioptric optical system) for imaging an irradiated
portion of the mask MA onto a target portion C (e.g. comprising one
or more dies) of the substrate W.
[0094] As here depicted, the apparatus is of a transmissive type
(i.e. has a transmissive mask). However, in general, it may also be
of a reflective type, for example (with a reflective mask).
Alternatively, the apparatus may employ another kind of patterning
means as an alternative to the use of a mask; examples include a
programmable mirror array or LCD matrix.
[0095] The source LA (e.g. a mercury lamp or excimer laser)
produces a beam of radiation. This beam is fed into an illumination
system (illuminator) IL, either directly or after having traversed
conditioning means, such as a beam expander Ex, for example. The
illuminator IL may comprise adjusting means AM for setting the
outer and/or inner radial extent (commonly referred to as
.sigma.-outer and .sigma.-inner, respectively) of the intensity
distribution in the beam. In addition, it will generally comprise
various other components, such as an integrator IN and a condenser
CO. In this way, the beam PB impinging on the mask MA has a desired
uniformity and intensity distribution in its cross-section.
[0096] It should be noted with regard to FIG. 8 that the source LA
may be within the housing of the lithographic projection apparatus
(as is often the case when the source LA is a mercury lamp, for
example), but that it may also be remote from the lithographic
projection apparatus, the radiation beam that it produces being led
into the apparatus (e.g. with the aid of suitable directing
mirrors); this latter scenario is often the case when the source LA
is an excimer laser (e.g. based on KrF, ArF or F.sub.2 lasing). The
current invention encompasses both of these scenarios.
[0097] The beam PB subsequently intercepts the mask MA, which is
held on a mask table MT. Having traversed the mask MA, the beam PB
passes through the lens PL, which focuses the beam PB onto a target
portion C of the substrate W. With the aid of the second
positioning means (and interferometric measuring means IF), the
substrate table WT can be moved accurately, e.g. so as to position
different target portions C in the path of the beam PB. Similarly,
the first positioning means can be used to accurately position the
mask MA with respect to the path of the beam PB, e.g. after
mechanical retrieval of the mask MA from a mask library, or during
a scan. In general, movement of the object tables MT, WT will be
realized with the aid of a long-stroke module (coarse positioning)
and a short-stroke module (fine positioning), which are not
explicitly depicted in FIG. 8. However, in the case of a wafer
stepper (as opposed to a step-and-scan tool) the mask table MT may
just be connected to a short stroke actuator, or may be fixed.
[0098] The depicted tool can be used in two different modes:
[0099] In step mode, the mask table MT is kept essentially
stationary, and an entire mask image is projected in one go (i.e. a
single "flash") onto a target portion C. The substrate table WT is
then shifted in the x and/or y directions so that a different
target portion C can be irradiated by the beam PB;
[0100] In scan mode, essentially the same scenario applies, except
that a given target portion C is not exposed in a single "flash".
Instead, the mask table MT is movable in a given direction (the
so-called "scan direction", e.g. the y direction) with a speed v,
so that the projection beam PB is caused to scan over a mask image;
concurrently, the substrate table WT is simultaneously moved in the
same or opposite direction at a speed V=Mv, in which M is the
magnification of the lens PL (typically, M=1/4 or 1/5). In this
manner, a relatively large target portion C can be exposed, without
having to compromise on resolution.
[0101] While specific details of various embodiments of the optical
proximity correction method utilizing adjustable serifs have been
disclosed herein, it is also clear that other variations are
possible. For example, many different rules for resizing or
repositioning serifs are possible. Indeed, the specifics of such
rules would vary from application to application. Accordingly, it
is not intended that the scope of the present invention be limited
to the foregoing examples. Clearly, variations of the specific
examples disclosed herein, as well as numerous additional rules,
are also possible.
[0102] As described above, the optical proximity correction method
utilizing adjustable serifs of the present invention provides
significant advantages over the prior art. Most importantly, the
optical proximity method of the present invention provides for
individual and flexible control of each serif so as to allow the
size and position of each serif to be adjustable. As such, it is
possible to modify the dimensions of serifs during the mask design
process so that serifs (that would otherwise be eliminated from the
design) satisfy minimum distance requirements. As a result, the
present invention minimizes the elimination of serifs from the mask
design, and the corresponding degradation in the pattern formed on
the wafer.
[0103] Although certain specific embodiments of the present
invention have been disclosed, it is noted that the present
invention may be embodied in other forms without departing from the
spirit or essential characteristics thereof. The present
embodiments are therefore to be considered in all respects as
illustrative and not restrictive, the scope of the invention being
indicated by the appended claims, and all changes that come within
the meaning and range of equivalency of the claims are therefore
intended to be embraced therein.
* * * * *